Controlled Synthesis Oi Gold-based Platinum Group Noble-metal Nanoparticles And Study On Their Surface Defect-based Applications

Because of the good electrical conductivity,high stability,excellent biocompatibility,and the unique optical and magnetic properties,the noble metal nanomaterials(NMs)are widely studied in the fields of surface enhanced Raman scattering(SERS),catalysis,biomedicine,sensing and other fields.It is worth mentioning that the physical and chemical properties of the noble metal NMs are not only affected by the size and morphology,but also closely related to the surface defects and the structures.For example,the construction of defects on the surface of noble metal NMs can often increase the number of active sites and improve the catalytic performance.Compared with the noble metal NMs of single component,the noble metal NMs with core-shell or alloy structure can achieve the synergistic effect between multiple metals,which usually showed higher catalytic activity and stabilityDue to the unique physical and chemical properties of gold nanoparticles(Au NPs),many Au NPs with varied types and functions have been successfully designed and synthesized by the researchers so far.However,in the process of synthesis and preparation of Au NPs,there are still some issues that need to be resolved.For instance,multi-step seeded growth method is always required in the preparation of high quality and large-sized Au NPs in order to avoid the adverse effect induced by the self-nucleation.And the experimental operation of the multi-step seeded growth is still complex to the researchers with no experience in material synthesis.Therefore,the application of Au NPs can be better extended to different fields if high quality and large-sized Au NPs with varied morphology can be fabricated by the one-step seeded growth method.It is known that the strain would be generated along the twin boundaries,thus the design and construction of twin defects in the NMs is recognized as one of the new strategies to improve its catalytic properties.While compared with the normal twin defects,the deformation twinned defects could generate larger strain and increase the roughness of the particle surface at the same time.Therefore,how to construct twin defects in the NMs is well worth studying.But as far as we know,there have been no reports of the fabrication of deformation twinned defects on the surface of Au NPs so far.Therefore,the design and preparation of Au NPs with deformation twinning would facilitate the further expansion of the application of Au-based NPs in the field of SERS and catalysis.In the core-shell(CS)structured gold-palladium(Au-Pd)bimetallic NPs,not only the thin Pd shell can improve the utilization of Pd metal and reduce the material costs,but also the electronic effect between the Pd shell and the Au core can improve the electrocatalytic performance.In addition,the Au core can improve the stability of the Au-Pd bimetallic NPs as well.However,the electrocatalytic performance of CS Au-Pd NPs are not very satisfactory at present,which because the tensile strain in the Pd shell induced by the lattice mismatch between the Au core and Pd shell.The tensile strain can upshift the d-band center of the Pd shell to close to the Fermi level and narrow the d-band width.Although the tensile strain is beneficial to the adsorption of the reactants in the catalytic process,it is not conductive to the timely release of the active sites,which further would reduce the reaction rate of the entire catalytic process.Therefore,in order to maximally improve the electrocatalytic performance of the CS Au-Pd NPs,the Pd shell should be given both of the proper adsorption ability towards the reactants and the proper desorption ability towards the intermediates(i.e.,the appropriate d-band center position and the d-band width).It is known that the compressive strain can downshift the d-band center and widen the d-band width of the Pd shell,and thus can enhance the desorption ability to the reaction intermediates.Therefore,the introduction of compressive strain into the CS Au-Pd NPs would be an effective strategy to further optimize the catalytic performance by balancing the adsorption and desorption ability of the catalysts.In addition,the high price as well as the low reserves of Platinum(Pt)metal in nature limited the extension and application of the Pt-based NMs.And in general,in the catalytic pathway of the hydrogen evolution reaction(HER)by the ordinary Pt-based noble metal NMs,the recombination of H and the desorption of H2 is regarded as the rate determining step.Therefore,the preparation of core-shell structured Pt-based NMs bearing both of the ultrathin shell contained Pt metal and compressive strain not only can reduce the amount of Pt metal in Pt-based NMs,but also can enhance the desorption ability to H2 of the Pt-based NMs,which further can improve the overall reaction rate of the HER.Based on the current issues mentioned above,the preparation and application of Au-based NPs with deformation twinned defects as main goal was studied in the thesis.Firstly,quasi-spherical deformation twinned Au NPs(QS Audt NPs)with vary-sizes and rough surface were synthesized by the one-step seeded growth method which optimized by the Cu2+ ions.And the SERS application of the QS Audt NPs were explored at the same time.Next,the morphological feature and composition of the deformation twinned defects in the QS Audt NPs were further investigated.Then,the QS Audt NPs were employed as the cores,and the Pd shells(about one atomic layer)with corrugated structure and compressive strain were conformally grown on the Au cores,and the electrocatalytic performance of the core-shell(CS)Audt-Pd NPs for the oxygen reduction reaction(ORR)and ethanol oxidation reaction(EtOR)were investigated.Finally,with the assistance of the transition Pd layer,the conformal growth of ultrathin alloyed PdPt shells on the QS Auat NPs were realized,and the core-alloyed shell(CAS)Audt-PdPt NPs were synthesized.And the morphology,the element composition and the electrocatalytic activity towards HER of the CAS Audt-PdPt NPs were explored preliminarily.This paper is divided into five chapters(Chapter 1 is the introduction),and the detailed research content is as follows:In chapter 2,firstly;uniformly,large-sized Au NPs including QS Audt NPs with average diameters of 70 to 196 nm and trisoctaheral(TOH)Au NPs with average diameters of 140 to 195 nm,were successfully synthesized by controlling the concentration of Cu2+ ions and the particle number of 3 nm Au-NP seeds,respectively,by the one-step seeded growth method which modified by the concentration-dependent Cu2+underpotential deposition(CuUpd)effect.At the same time,the rough surface and the deformation twinned defects in the QS Audt NPs were preliminary characterized Secondly,the mechanism of Cu2+ ions was investigated by characterization of the changes in the morphology and size of the corresponding intermediated Au NPs during the growth process.And on the one hand,the morphology control of the Au seeds can be achieved by the Cu2+ions at the initial growth stage;on the other hand,the Cu2+ions can realize the conformal growth of Au NPs at the ripening stage.Finally,the application of the prepared QS Audt NPs with rough surface in the field of SERS was explored.And compared with the similar-sized TOH Au NPs with sharp tips,the as-prepared QS Audt NPs with rough surface bear a better SERS performance for physically-adsorbed probes(crystal violet,CV).And due to the specific Au atom arrangement induced by the deformation twinned defects on the surface of QS Audt NPs achieved the distinction of nitrile and isonitrile group was realized by the Raman spectra of the QS Audt NPs substrate.In Chapter 3,firstly,the deformation twinned defects and the compressive strain induced by the defects in the QS Audt NPs were further characterized.And then,CS Audr-Pd NPs with corrugated structured Pd shells were successfully synthesized by the conformal growth of the ultrathin Pd shells on the surface of the QS Audt NPs.At the same time,the surface defects and the element composition of CS Audr-Pd NPs were characterized.The compressive strain in the Pd shell was confirmed by different ways,and the strain is the strongest when the Pd shell is one atomic layer.Finally,taking the ORR and the EtOR in alkaline condition as examples,the electrocatalytic performance of the commercial catalysts and the as-prepared CS Au-Pd NPs with one atomic layer of Pd shell and different type of Au core(single crystalline,polycrystalline and deformation twinning)were investigated.Thanks to the synergistic effect of the compressive strain effect and the electronic effect,the CS Audt-Pd NPs bear the enhanced desorption ability to intermediates and the good adsorption ability to reactants simultaneously.Therefore,the half-wave potential and the kinetic current density of the ORR catalysed by the CS Audt-Pd NPs reached up to 0.93 V and 12.59 mA cm-2,which are significantly better than that of the CS Aus-Pd NPs(0.87 V and 1.83 mA cm-2),CS Aup-Pd NPs(0.89 V and 4.22 mA cm-2)and the commercial Pt/C catalysts(0.89 V and 6.9 mA cm-2),respectively.And in the EtOR under alkaline condition,the specific activity of the CS Audr-Pd NPs(6.03 mA cm-2)is also much higher than that of the CS Aus-Pd NPs(1.03 mA cm-2),CS Aup-Pd NPs(2.2 mA cm-2),and the commercial Pd/C catalysts(0.48 mA cm-2).Moreover,in the accelerate durability test,the CS Audt-Pd NPs also showed a better stability compared with the commercial Pt/C or Pd/C catalysts,respcetively.In Chapter 4,firstly,the core-alloyed shell(CAS)Audt-PdPt NPs were synthesized by the conformal growth of ultrathin PdPt alloyed shell on the surface of the QS Audt NPs with the assistance of the transition Pd layer.And then,the surface defects and the element composition of the CAS Audt-PdPt NPs were characterized preliminarily.And the alloyed PdPt shell bear the corrugated structure same as the Pd shell in CS Audt-Pd NPs,thus the alloyed PdPt shell in CAS Audt-PdPt NPs should bear compressive strain as well.Finally,the commercial Pt/C catalysts and the as-prepared CAS Au-PdPt NPs with ultrathin PdPt alloyed shell and different type of Au core(single crystalline,polycrystalline and deformation twinning)were employed as the catalysts for the HER in acid condition.And due to the synergistic of the electronic effect between Au,Pd and Pt metals and the compressive strain effect,the CAS Audt-PdPt NPs(η10mA cm-2=15 mV)showed excellent electrocatalytic performance towards HER compared with CAS Aus-PdPt NPs(28 mV),CAS Aup-PdPt NPs(25 mV),and the commercial Pt/C catalysts(35 mV).In addition,after the accelerate stability test,the overpotential of the commercial Pt/C catalysts decreased by 10 mV while that of the CAS Audt-PdPt NPs only decreased by 4 mV,which indicates the very good stability of the sample.Chapter 5 is a summary and prospect of current research work.In summary,in this thesis,firstly,large-sized QS Au NPs with deformation twinned defects(Audt NPs)were successfully synthesized.And then the Audt NPs were employed as the core,CS Audt-Pd NPs with one atomic layer of Pd shell,and CAS Audt-PdPt NPs with ultrathin alloyed PdPt shell were designed and synthesized.Finally,the performance of the as-prepared Au-based NPs in the fields of SERS and electrocatalysis were investigated.